The invention relates to a charge recycling amplifier within a CMOS imaging device having improved dynamic range.
One technique to increase dynamic range of a CMOS active pixel imager is to amplify pixel signals using two separate signal gains. The first applied gain is usually smaller than the second. The two amplified pixel signals are later combined together to produce the actual pixel output signal.
Since the currently adopted column parallel architecture uses a destructive read-out scheme, multiple exposures of the CMOS imager pixel array are required to capture pixel signals for each of the applied gain values. Sampling multiple exposures is a time consuming process, limiting the speed of image capture.
The invention provides a charge recycling amplifier for use in a CMOS imaging device so that only one exposure of the array is required while still producing a pixel signal subject to two different gain values. The charge recycling amplifier comprises a differential amplifier; a pair of input capacitors connected to the inputs of the differential amplifier; first and second pairs of feedback capacitors where each are connected between the input and output of the differential amplifier; a crowbar switch for forcing a charge present on the input capacitors through the differential amplifier; and first and second pairs of gain switches connected to the first and second pairs of feedback capacitors respectively, for setting the gain of the charge recycling amplifier and at least one switch for recycling output charge on a pair of feedback capacitors back to the input of the differential amplifier. A pixel signal presented to the amplifier is first amplified with a first gain setting and then the amplified signal is amplified again with a second gain setting.
In yet another aspect of the invention, a method of operating the above components is disclosed.
These and other features and advantages of the invention will be more clearly seen from the following detailed description which is provided in connection with the accompanying drawings.